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The realization of a transparent back buffer layer would be a significant boon for the CdTe community. Simulations presented here reveal a substantial performance increase in ultrathin cells through incorporation of a back buffer, as well as high bifaciality if transparent contacts are assumed. This project aims to deposit a p-type transparent conducting oxide (TCO) at the back of superstrate CdSeTe/CdTe cells to improve band bending and interface recombination at the rear of the cell, and ultimately to bring high performing bifacial cells to fruition. Four such oxides were sputter deposited to create a collection of functional bifacial cells. Back surface passivation was evaluated with rear-excitation time-resolved photoluminescence (TRPL). Several promising materials yielded rear-excitation lifetimes >2 ns, indicating effective passivation.

The passivating effects of MgO have been studied by integrating into an existing superstrate CdTe thin film solar cell device architecture. When implemented as an emitter to replace the typical MZO, the device performance was below par, but improved PL and TRPL was detected with significant carrier lifetimes in comparison to the MZO devices. However, when integrated at the back, the PL signal was detected upon the illumination of excitation laser from the back. This has opened the possibility of probing the back of the CdTe thin film solar cells with transparent back contact with improved back passivation and a bifacial device structure. Further, the TRPL measurements suggests that the MgO shows passivating effects to a CdTe surface irrespective of method of deposition implemented.

The doping effect of arsenic (As) concentration in the CdTe absorber has been studied by using the first principles method. Atomistic modeling based on density functional theory (DFT-1/2) and green’s function (GF) was utilized to simulate a first order approximated model of As doped CdTe surface. The band alignment results calculated using DFT-1/2 revealed that higher concentration of As doping at CdTe surface is required to obtain favorable band bending for hole charge carrier transport. Based on the results obtained from DFT models, CdTe solar cells were fabricated with two different As dopant concentrations. The experimental finding corroborates the theoretical result and provides a future pathway of using DFT simulations as a precursor in guiding the CdTe device fabrication experiments.